The invention relates generally to the fiber optic communications, and more particularly to optical packaging techniques used to align laser sources to optical fibers and other types of waveguides.
Optical fiber communications has generally replaced electrical links over long distances in the past few decades. In more recent past, optical links are being used at shorter distances to connect servers to switches and for datacenters. In the future it is expected as data rates increase and costs of optics decreases that optics will diffuse into computers and the connections within a machine or between processors will be optical. (see for example Kash et al. SPIE Photonics West conference 2009 and references cited therein, the disclosures of which are incorporated by reference herein)
A challenge of fiber optics has been that packaging and alignment processes are considerably more difficult than electrical wiring. The advantages are the greater bandwidth and reduced degradation of the signal with distance. At 10 Gb/s data rates, for the signal to travel more than 100-300 m in a fiber, single mode fiber is generally needed, with a typical mode size of about 8 microns. Laser sources typically have a mode size of only a few microns. Thus the alignment between the laser and the fiber through the intermediate optics generally has to be very high precision, and tolerances on the order of a tenth of a micron are typically required. One great advantage of single mode fiber is that multiple wavelengths can be coupled simultaneously to get a parallel link through a single fiber. Thus a 100 Gb/s signal can be sent through a single mode fiber for many kilometers by using ten channels of 10 Gb/s each, with every lane at a different wavelength.
As an alternative, when distances are on the order of 100 m or less, multimode fiber and multimode vertical cavity lasers are often used. In this case the core size in the fiber is much larger, at about 50 um, and tolerances can be substantially looser. However, the reach is limited as different modes of the fiber travel at different speeds and it is becomes more difficult to transmit multiple wavelength simultaneously.
As bandwidth requirements increase, there is increased parallelism in both single mode and multimode fiber links. In single mode systems, parallel channels can be obtained easily by adding wavelengths to the same fiber. In multimode systems, additional fibers generally are added to form a fiber ribbon. Parallel ribbon fibers are of course quite expensive and connectors with 24 fibers inside are complicated to make, even if they use multimode fiber with looser alignment tolerance.
There has been considerable work in the industry on different techniques of loosening the alignment tolerance in single mode systems. However, none is very effective, especially if multiple sources are coupled into the same fiber. In these cases there are multiple single mode alignments that occur in the same package.
The simplest way to loosen the tolerances slightly is to fabricate a laser with a bigger optical mode. The technique most commonly used is to have a tapered section at the output of the laser where the optical mode is expanded. This makes the laser mode roughly the same size as the optical fiber or waveguide mode and the alignment tolerance increases from about a quarter micron to about a micron. The disadvantage of this technique is that the fabrication of the laser or semiconductor source becomes more complex, raising the cost. There is also some sacrifice in the performance of laser. In addition, the effect of a laser with a slightly larger optical mode is not that dramatic. One micron alignment tolerance is better than a quarter of a micron, however, it is still not amenable to low cost packaging techniques.
Another technique is to etch the facet of the laser and add a passive silica waveguide. The laser is bonded upside down to a planar lightwave circuit (PLC) that has waveguides built in. The passive waveguide in the laser source and the waveguide in the PLC are matched in effective index, and with a slight taper, all the power can theoretically transfer from the laser source into the single mode waveguide underneath. This loosens the tolerance in the die bonding process to about 5 um, allowing the use of some standard packaging and diebonding equipment. The issue with this technique is that the laser chips become tremendously more complicated. One has to etch a facet and through epitaxial and lithographic processes, align a passive waveguide to the semiconductor waveguide. Such lasers are highly customized and there is an unavoidable optical loss between the laser waveguide and the passive waveguide formed next to it.
MEMS with active rather than passive alignment has also been used to align lasers and waveguides. Alignment may be performed with a MEMS mirror with alignment maintained by a control loop. However, the feedback loop has to be maintained during operation, requiring that the high voltage control electronics outside of the package stay active during operation.
There have been some proposals of MEMS active alignment techniques for switches and alignment of arrays. Some have moving waveguides (E. Ollier, “1\×8 Micromechanical Switches based on Moving Waveguides,” in Proc. 2000 IEEE/LEOS Int. Conf. Opt. MEMS Kauai, H I, August 2000, pp. 39-40.), some have torsional mirrors (MEMS optical switches, Tze-Wei Yeow; Law, K. L. E.; Goldenberg, A. Communications Magazine, IEEE Volume 39, Issue 11, November 2001 Page(s):158-163) and some with lenses on an x-y stage (MEMS packaging for micro mirror switches, Long-Sun Huang; Shi-Sheng Lee; Motamedi, E.; Wu, M. C.; Kim, C.-J. Electronic Components & Technology Conference, 1998. 48th IEEE Volume, Issue, 25-28 May 1998 Page(s):592-597) (all of which are incorporated by reference herein). However, all of these approaches are complex and difficult to apply, for example, to PLCs.